1. Field of the Invention
The present invention relates to electromagnetic exciters that may be incorporated into mobile communications devices such as cellular phones and personal digital assistants (PDAs) to inform a user of an incoming call by vibrating, for example, the housing of such a mobile communication device.
2. Description of the Related Arts
Conventionally, mobile communication devices, e.g. cellular phones and PDAs, are arranged to inform the user of an incoming call by selectively generating beep sound or melody sound, or vibrating the housing of the device instead of producing sound. For this purpose, the conventional practice is to employ a structure incorporating a combination of a sound-generating speaker categorized as a microspeaker and a vibrator that generates vibration by rotating an eccentric weight with a small motor. Incorporating both the speaker and the vibrator as stated above, however, is disadvantageous from the viewpoint of achieving size and cost reduction of the device. Under these circumstances, use has recently been made of an electromagnetic exciter that can generate both sound and vibration by itself.
The inventor of this application developed an electromagnetic exciter 51 as shown in FIGS. 6 to 8 (Japanese Patent Application No. 2005-014919).
FIG. 6 is a vertical sectional view showing the right-hand half of the electromagnetic exciter 51. The left-hand half of the electromagnetic exciter 51 is substantially symmetric to the right-hand half with respect to the center axis C of the electromagnetic exciter 51. The electromagnetic exciter 51 has a magnetic circuit assembly 62 including a pan-shaped outer yoke 8, a circular columnar permanent magnet 9, and an inner yoke 10. The permanent magnet 9 and the inner yoke 10 are successively stacked in the outer yoke 8. The magnetic circuit assembly 62 is suspended within a cylindrical casing 52 by a suspension 63. A top plate 52a is provided to cover the upper end opening of the casing 52. A voice coil 5 is secured to the top plate 52a. A ring-shaped weight 61 is secured to the outer peripheral portion of the outer yoke 8. A back member 4 made of a dust-proof mesh or metal plate, for example, is provided to cover the lower end opening of the casing 52. The weight 61 is used to reduce the natural frequency of the magnetic circuit assembly 62 and generally formed by using a high-density material, e.g. tungsten. A magnetic gap g is formed between the inner yoke 10 and the upper end of the outer yoke 8. The voice coil 5 is positioned in the magnetic gap g.
FIG. 7 is a plan view of the suspension 63. The suspension 63 is formed by stamping a leaf spring member. The suspension 63 has an inner ring 63a and an outer ring 63b, which are in concentric relation to each other, and a pair of arcuate arms 63c that interconnect the inner and outer rings 63a and 63b. As shown in FIG. 6, the inner ring 63a of the suspension 63 is secured to the outer yoke 8 of the magnetic circuit assembly 62. The outer ring 63b is secured to the casing 52 by being clamped between an outer cylindrical portion 52b and an inner cylindrical portion 52c of the casing 52. The spacing between the outer ring 63b and each arm 63c is set larger than the spacing between the arm 63c and the inner ring 63a. Arcuate projections provided on the outer peripheral edge of the weight 61 are inserted between the outer ring 63b and the arms 63c. The purpose of this arrangement is to increase the mass of the weight 61 by effectively utilizing the space in the electromagnetic exciter 51.
The electromagnetic exciter 51 is secured to an electronic device such as a cellular phone by applying a double-sided adhesive tape to the top plate 52a and attaching the adhesive tape to the housing of the cellular phone, for example. When the voice coil 5 is excited by an input driving signal, the voice coil 5 and the magnetic circuit assembly 62 vibrate relative to each other by interaction between the coil 5 and the magnet 9. When the driving signal is of a relatively high frequency in an audio-frequency region, the voice coil 5 mainly vibrates, causing the housing of the electronic device to vibrate through the top plate 52a. When the frequency of the driving signal is low, the magnetic circuit assembly 62 mainly vibrates, and this vibration causes the housing of the electronic device to vibrate through the suspension 63 and the casing 52.
In the electromagnetic exciter 51 shown in FIG. 6, the magnetic circuit assembly 62 is supportingly connected to the casing 52 by the suspension 63 arranged as stated above. Therefore, when the magnetic circuit assembly 62 is vibrated, the magnetic circuit assembly is moved up and down, together with the inner ring 63a, relative to the outer ring 63b, which is secured to the casing 52. Accordingly, the arcuate arms 63c of the suspension 63 arcuately bend about the respective outer joints 63d, so that the arms 63c are displaced greatly near the inner joints 63e and displaced a little near the outer joints 63d. Since the weight 61 is moved up and down together with the inner ring 63a, the weight is likely to be brought into contact with the arms 63c near the outer joints 63d. 
Under the circumstances, it is conventional that a clearance h1 is provided between the magnetic circuit assembly 62 and the top plate 52a so that the magnetic circuit assembly 62 will not contact the top plate 52a even when the magnetic circuit assembly 62 vibrates with a maximum amplitude relative to the top plate 52a. Further, the clearance between the weight 61 and the arms 63c of the suspension 63 is set to a uniform size h2 that is sufficiently large to avoid contact between the weight 61 and the arms 63c near the outer joints 63d. 
Meanwhile, in order to increase the acceleration of vibration occurring in the electromagnetic exciter 51, the weight 61 needs to be made as large as possible.